The waste water (flushing water, rinse water and spent acid) from a mixed acid pickling bath, for example, is usually sent to a neutralization stage where it is neutralized with milk of lime. The resulting mixture is then filtered and discharged. The fluorides and metals remain in the filter cake. The entire nitrate content of the filtrate, on the other hand, enters the main output. Since the nitrate content in the waste water is now being subjected to more and more stringent regulations, “nitrate-free” pickling has been suggested which uses sulfuric acid, hydrofluoric acid or hydrogen peroxide, for example, as pickling acid medium.
The disadvantage of this, however, is that the surface quality of the treated metal is not quite as good and the amount of sludge produced after neutralization is increased by 100%, involving higher costs for disposal. Another solution that has been suggested for reducing nitrate content is pyrohydrolytic treatment of the neutralized waste acid, which results in a 70-90% reduction in nitrate content for the used acid. However the nitrate load from the flushing water remains in the waste water. Accordingly, there is a continuing need in the industry for improved processing to remove nitrates from waste water.